Design Guide For Sizing Vertical Oil Treaters

23 September 2013

Posted by ankur2061 in Technical

Today's blog entry provides some guidelines for sizing of vertical oil treaters. The procedures described in this blog entry do not give the overall dimensions of the treater, which may include inlet gas separation and free-water knockout sections, dependent on the upstream separation vessels. However, they do provide a method for specifying heat input required and a minimum size for the coalescing section (where the treating actually takes place), and provide the design engineer with tools to evaluate specific vendor proposals.

Before going on to the sizing of the vertical treater let us understand the specific application of a vertical oil treater. The most common type of single-well onshore treater is the vertical treater, shown in the attached sketch. Vertical treaters are recommended where sand or other solid sediments are considered a potential problem.

Let us move on to the sizing equations for a vertical oil treater

Heat Input Requirements

Metric Units

q = 1100*Q_o*ΔT*[0.5*(SG)_o + (SG)_w*W_in /(100-W_in)] ----------1(a)

USC Units

q = 15*Q_o*ΔT*[0.5*(SG)_o + (SG)_w*W_in /(100-W_in)] -------1('b)

where:

q = Heat input, W (Btu/hr)
Q_o = Oil flowrate, m³/hr (BOPD)
ΔT = Temperature increase, ⁰C(⁰F)
(SG)_o = Oil specific gravity relative to water
(SG)_w = Water specific gravity
W_in = Inlet percent water cut, percent

Water Droplet Size

Metric Units
If µ_o< 7.0 x 10 ^–2Pa s, then d_m = 1361*W_c^0.33*µ_o^0.25 -----------2(a)
If µ_o≥ 7.0 x 10 ^–2Pa s, then d_m = 700*W_c^0.33 ------------2('b)

USC Units
If µ_o< 70cP, then d_m = 242*W_c^0.33*µ_o^0.25 -------------2('c)
If µ_o≥ 70cP, then d_m = 700*W_c^0.33 -------------------2(d)

µ_o = Oil viscosity, Pa s (cP)
d_m = water droplet diameter, microns
W_c = Outlet Water Cut, percent

Vertical-Settling Time Equation

Metric Units
d = 806,000*(F*Q_o*µ_o / (ΔSG*d_m²))^0.5 -------------------3(a)

USC Units
d = 81.8*(F*Q_o*µ_o / (ΔSG*d_m²))^0.5 ---------------------3('b)

where:
d = vessel minimum diameter, mm (inch)
Q_o = Oil flow rate, m³/h (BOPD)
µ_o = Oil viscosity, Pa s (cP)
ΔSG = Difference in specific gravity of oil & water
d_m = water droplet diameter, microns
F = short-circuiting factor
=1, with very good flow distribution and smaller than 1220 mm (48 inch) diameter
or
= d/1220 (d/48) for treaters over 1220 mm (48 inch) diameter
Note:
First solve Eqn 3 using F=1. If the value of d is less than or equal to 1220 mm (48 inch), this is the final answer. If the value of d is greater than 1220 mm (48 inch), then substituting F = d/1220 (F = d/48) into above Eqn 3 gives the following modified equation:

Metric Units
d = 5.33*10⁸*Q_o*µ_o / (ΔSG*d_m²) ----------------4(a)

USC Units
d = 139*Q_o*µ_o / (ΔSG*d_m²) --------------------4('b)

where:
d > 1220 mm (48 inch)

Note:
The height of the coalescing section for a vertical treater plays no part in the settling equation. The cross-sectional area of flow for upward velocity of the oil is a function of the diameter of the vessel alone.

Vertical-Retention Time Equation
The oil should be held at a temperature for a specific period of time to enable demulsifying the water-in-oil emulsion. This time can be best obtained by a laboratory bottle test. However, in absence of such data, 20-30 minutes is a good starting point.

Metric Units
d²*h = 2.12*10⁷*F*(t_r)_o*Q_o -----------------5(a)

USC Units
d²*h = 8.6*F*(t_r)_o*Q_o ------------------------5('b)
where:
h = height of coalescing section, mm (inch)
(t_r)_o = Oil retention time, minutes (minutes)

Re-arranging Eqn 5 by substituting F, we get

Metric Units
If d ≤ 1220 mm (F = 1) then
h = 2.12*10⁷*(t_r)_o*Q_o / (d²) ---------------------6(a)

If d > 1220 mm (F = d/1220) then
h = 1.75*10⁴*(t_r)_o*Q_o / (d) ---------------------6('b)

USC Units
If d ≤ 48 inch (F = 1) then
h = 8.6*(t_r)_o*Q_o / (d²) --------------------------6('c)

If d >48 inch (F = d/48) then
h = (t_r)_o*Q_o / (5.58*d) -------------------------6(d)

Note:
Part of the overall vessel height is required to provide for water retention. The removal of oil from the water is not of primary concern.

Water-Retention Time Equation

Metric Units
h_w = 2.12*10⁷*(t_r)_w*Q_w / (d²) ----------------7(a)

USC Units

h_w = 8.6*(t_r)_w*Q_w / (d²) -----------------------7('b)

where:
h_w = height of water, mm (inch)
(t_r)_w = water retention time, minutes (minutes)
Q_w = Water flow rate, m³/h (BOPD)

This concludes today's blog entry on sizing or adequacy check of vertical oil treaters, Readers are free to put up questions and I will try my best to answer them.

Regards,
Ankur

Attached Thumbnails

7 Comments

gecos

Sep 25 2013 02:08 AM

nice resume, thank you.

benabed

Sep 29 2013 02:43 PM

Dear ANKUR,

This is the time a heared about a vertical oil treater could you please tell me the purpose of the oil treater and the operational principle.Because the attached skethh didn't explain how the oil is separated from water.

THANK YOU IN ADVANCE.

ankur2061

Sep 29 2013 10:47 PM

Dear ANKUR,

This is the time a heared about a vertical oil treater could you please tell me the purpose of the oil treater and the operational principle.Because the attached skethh didn't explain how the oil is separated from water.

THANK YOU IN ADVANCE.

Benabed,

As the name implies it is an "Oil Treater". Oil treaters separate "Basic Sediment & Water" (BS&W) from oil and not oil-from-water. As I have already mentioned removal of oil from water is not of primary concern in an oil treater. The coalescing section as shown in the sketch allows the colaescence of water droplets from the oil based on the difference in gravity of oil and water and settling velocity for water droplets.

Hope this clarifies your doubt.

Regards,

Ankur.

FRED2

Oct 16 2013 11:00 AM

Ankur,

Thanks for the fundamental principles of an oil treater . A spread sheet will be highly appreciated.

tarang.suthar

Oct 21 2013 10:39 AM

Dear sir

I have qurrey regarding pipe line size?

I have either flow rate of line size 38 od and 35.6 mm ID and 6 bar(g) Compressed air without assumed Velocity, If you have find out than give me the steps and excel sheet if you have.